It is known to compress pulse code modulated (PCM) voice signals according to CCITT recommendation G.727 known as embedded ADPCM, by grouping the bits of a succession of voice samples into a predetermined number of blocks according to the significance of these bits. For example, five bit samples taken at a rate of 40 kbps over an interval of 16 milliseconds will produce 5 blocks of 128 bits each. The blocks containing the most significant bits and the next most significant bits are known as core blocks and the other blocks containing the less significant bits are known as enhancement blocks. Furthermore, according to CCITT recommendation G.764 known as the packetised voice protocols, it has been proposed to assemble the core blocks and associated enhancement blocks into a packet for transmission across a packet switched network, each packet including an address field by which it is routed through the network and a block dropping indicator which indicates which of the enhancement blocks can be discarded during periods of congestion in the network.
Discarding one or more enhancement blocks reduces the size of the packet and thereby relieves congestion. The quality of the voice signal derived from the shorter packets is reduced, but only in a progressive and manageable manner by first discarding the enhancement block containing the least significant bits, followed by the enhancement blocks of progressively more significant bits until only the core blocks remain.
A difficulty with this known system for controlling congestion in packet voice switching networks is that extra processing is involved at each node in analyzing the header of each packet and processing the packet to shorten it when required.